I don't know how bad this is where you are buy everyone drinks diet coke where i am

Essentially to lost weight you need to eat less, and to gain muscle you need to work out. How much of each is dependent on what you want to achieve. Don't expect to have the body of someone who works harder or diet harder.

Honestly, I can recommend 10 back exercises but in terms of bone strain - because exercises use muscle and bone - they'll all be equivalent to a lighter deadlift, possibly at higher reps.

I could be wrong. Probably am. There should be scientfic research on this. Go get access to a science database and check what their results on bone weakness on accutane is, then report back on how much more weakened it is. It could be less than people think, or certain movements only. In the end you're asking us to circumvent a doctor's advice. No heavy lifting. Do your usual weight routine for more reps.